Delay circuits for delaying a signal for a desired time period are well known. Most of these known delay circuits typically may be used in a variety of applications. One such application is the testing of integrated circuits. A crucial step in the manufacturing of integrated circuits is the testing of those circuits after device fabrication. As with each of the steps involved in integrated circuit manufacture, it is advantageous to perform this testing with as little cost as possible. Therefore, high speed, automated integrated circuit testers are necessary.
As integrated circuits become more complex and require more pins for interconnection to circuit boards and the like, the apparatus necessary for testing the integrated circuits becomes much more complex. Current DC testers have multiple force and measure units, each applying a predetermined forcing condition of voltage and/or current to a pin of a test head and measuring the response thereto. Each pin of the test head is connected to a pin of the integrated circuit under test. Typically, a minicomputer contains the testing and measuring routines and is responsible for managing the testing process.
Current AC testers provide test signals from the tester to the integrated circuit under test that are delayed for providing setup and hold time, timing edges, etc. The delay circuit must be accurate, have a large adjustable range and be able to change delays in a few nanoseconds. Portions of previously known delay circuits were on chip but required external comparators and ramp circuits and did not provide the desired accuracy, range and quick change rate required for efficient high speed testing.
Thus, what is needed is an accurate on-chip delay circuit that has a large adjustable range.